1. Field of the Invention
The present invention relates generally to methods and apparatus for use in chemical milling of metallic materials. More particularly the invention concerns a unique method and apparatus for automatically cutting a maskant material, such as a thin film of plastic, which has been applied to the metal workpiece to be milled.
2. Discussion of the Prior Art
Chemical milling may be defined as a process of etching the surfaces to be milled by chemical attack. The techniques for chemical milling of metallic workpieces are well known and have proven particularly useful in the past for applications wherein it is desired to remove specific amounts of material in predefined areas of aluminum, magnesium, titanium or steel sheet material after the sheet has been either rolled or stretch formed. As a practical matter, it is not feasible to mechanically mill large sections of sheet material, and particularly sheet material having a compound curved surface, due to equipment limitations and great expense. However, in many applications, including aerospace applications, where part weight and wall thickness tolerances are critical, precision milling of large sheet metal components is frequently required. Chemical milling has proven quite valuable and is widely used in such applications.
The standard approach followed in the past in chemical milling sheet workpieces to a uniform wall thickness was to first measure the wall thickness of the part at a multiplicity of points. The wall thickness data thus obtained was then used to draw contour lines on the surface of the part which represented regions of greater and lesser wall thickness.
After the contour lines were drawn on the surface of the part, the next step in the prior art procedures was to cover the surface of the part with a thin film of vinyl plastic, gelatin, rubber base material, or other etch-proof film, or maskant. This was done by spraying, painting, dipping or otherwise applying the maskant to the surface of the part. Due to the substantial transparency of the maskant, the contour lines drawn on the part surface remained visible. Next, using a sharp knife or razor blade, a portion of the maskant was cut away by hand as, for example, along the contour lines of an area of greater wall thickness. The part was then immersed into the etching bath which comprised acid, a suitable caustic, or other chemical attacking means. Since the maskant protected all the surface save the unprotected area, only this area would be attacked by the chemical and would be milled away. Successive steps of cutting away the maskant from other portions of the part, reimmersing of the part into the etching bath and continued gaging of the etched areas permitted precise milling of the surface of the part to a desired uniform wall thickness. A typical prior art technique for chemical milling using a polyvinyl maskant is described in U.S. Pat. No. 2,739,047 issued to Manuel C. Sanz.
Particularly with large parts, the time required to gage and mark the surface areas to be etched was highly labor intensive, often involving many man hours. Similarly, the time required to then manually cut the maskant was inordinately large.
One of the most significant advancements in chemical milling over the prior techniques as described in the preceding paragraphs is disclosed in co-pending application Ser. No. 06/542,790, now Pat. No. 4,523,973, filed by the present inventor. In this application there is disclosed an improved and highly unique method and apparatus for automatically measuring, scribing, chemically milling and inspecting sheet metal workpieces. In the preferred form of the apparatus of the invention described in this application, maskant cutting is accomplished by a laser technique.
The present invention contemplates the use of equipment similar to that described in the aforementioned co-pending application, but provides a unique alternative mechanism for cutting the maskant material. More particularly the apparatus of the present invention, instead of embodying a laser device for cutting the maskant, involves the use of a novel maskant cutting apparatus which embodies a sharp cutting blade adapted to be automatically advanced along a predetermined contour line in a constant, controlled light contact with the surface of the metal workpiece.
The maskant cutting apparatus of the present invention can also be used in conjunction with simplified motion generating devices which function to move the tool holder only in first and second directions relative to the workpiece.